A.(1). Field of the Invention
The invention relates to a speech analysis system of a type wherein the amplitude spectrum of a speech signal is analyzed by regularly selecting time segments of the speech signal, by determining from each time segment a sequence of spectrum components which constitute the discrete Fourier transform of samples of the speech signal and by deriving in each time segment the positions of the significant peaks in the spectrum from the sequence of spectrum components.
The significant peak positions constitute the input data for a subsequent section of the speech analysis system for determining the pitch of the speech signal.
A.(2). Description of the Prior Art
A speech analysis system which utilizes a FFT-transform and is of the type described sub A(1) is disclosed in IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP, No. 4, August 1978, pp. 358-365. Therein the pitch is determined from the spacings between the peaks in the spectrum.
An article in Philips Technical Review, Vol. 5, No. 10, October 1940, pp. 286-294 shows already that the pitch is not correlated with the spacing between the harmonics but with the periodicity of the collective mode of oscillation of the component harmonics.
In the thesis by E. de Boer entitled: On the "residue" in hearing, University of Amsterdam, 1956, a m.s.e. (mean-square-error) criterion is used to determine a probable value of the pitch associated with a sequence of spectrum components of which the so-called "harmonic numbers" are known, which are the numbers of the nearest harmonics of the fundamental tone.
In an article in the Journal of the Acoustic Society of America, Vol. 54, no. 6, June 1973, pages 1496-1516, it is shown that the above-mentioned m.s.e. criterion and the "maximum likelihood" criterion developed in this article and based on psycho-physical phenomena result in the same estimate of the pitch.
In the analysis of speech signals originating from sources such as telephone lines not only the problem occurs that the fundamental tone itself may be absent but also that noise components are introduced, which may considerably affect the result of pitch determination.